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The Seashore

Sand Dunes

Destructive Waves

In waves, individual water molecules move in circles . There is little forward motion of the water, it is the wave that moves forward. If you lose your football in the sea (assuming it's not blown somewhere by the wind) the ball will stay roughly in the same place moving up and down as the waves pass. As waves approach shallow water (about two thirds the wave height) the circular movement of the water molecules is impeded by friction from the bottom. The water at the top of the circle therefore is not supported by more water (as it would be if the wave was moving forwards) and it moves forwards into thin air. A crest forms which builds higher and higher until it topples over and breaks on the shore. All the circular motion is now converted to forward motion and the water rushes up the beach. This is called the SWASH of the wave. The water rushes up the beach and then with much less energy drains back down the slope. This is called the BACKWASH of the wave.

The swash of the wave tends to push material up the shore and the backwash tends to wash it back again. If there are a lot of waves (high frequency) per unit time backwash will tend to meet the swash of the next wave. This will spoil the forward motion, so less material will be pushed up the beach. The backwash will continue unhindered and there will be a net loss of material from the beach. Hence the term destructive waves. The difference between destructive and constructive waves is their frequency.


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